1. Field of the Invention
The present invention relates to a surface emitting laser and a manufacturing method therefor.
2. Description of the Related Art
In a vertical cavity surface emitting laser (VCSEL) which is a kind of a surface emitting laser, light can be taken out in a direction perpendicular to a surface of a substrate. Therefore, a two-dimensional array can be formed with ease just by changing a mask pattern when a device is formed.
By parallel treatment using multiple beams emitted from the two-dimensional array, a higher density and a higher speed are made possible, and industrial application thereof to various fields such as optical communications is expected.
For example, when a surface emitting laser array is used as an exposure light source of an electro-photographic printer, a higher density and a higher speed of a printing process using multiple beams are made possible.
In such a printing process in electro-photography, because a stable and minute laser spot needs to be formed on a photosensitive drum, the laser is required to operate with stability in a single transverse mode and in a single longitudinal mode.
Recently, for higher performance of a surface emitting laser, a method of injecting current using selective oxidation as described in the following has been developed.
In the selective oxidation, an AlAs layer or an AlGaAs layer having a high Al composition ratio, for example, having an Al composition ratio of 98% is provided in a multilayer reflecting mirror. By its selective oxidation in a water vapor atmosphere at high temperature, a current confining structure is formed such that current is injected only into a region in which the current injection is necessary.
However, the above-mentioned selective oxidation for forming the current confining structure is not desirable from the viewpoint of operation in the single transverse mode.
More specifically, a difference in the refractive index which is larger than necessary is caused due to the existence of the oxide layer, which in turn causes a high order transverse mode.
Measures taken against this include a method of preventing confinement of the high order transverse mode by making the diameter of the light emitting region as small as about 3 μm to attain single transverse mode oscillation. However, in such a method, because the light emitting region becomes smaller, output per device is significantly lowered.
Further, because current is injected into a minute light emitting region, the current density becomes higher, which is a cause of increase in device resistance, shortened device life, and the like.
In view of the above, conventionally, methods of attaining single transverse mode oscillation while maintaining a light emitting region being large to some extent by intentionally introducing a loss difference between a fundamental transverse mode and the high order transverse mode are reviewed.
As one of such methods, H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999) discloses a method of making a high order transverse mode loss larger than a fundamental transverse mode loss by forming a stepped structure on a light emitting surface of a surface emitting laser device.
FIG. 5 is a schematic sectional view of a surface emitting laser in which a surface relief structure is formed according to the method.
It is to be noted that a structure having a step in a light emitting region of a light emitting surface of a reflecting mirror as described above is hereinafter referred to as a surface relief structure.
By the way, when a loss difference is given to respective optical modes of a VCSEL using the surface relief structure, horizontal alignment between the surface relief structure and the current confining structure is important.
More specifically, when only fundamental transverse mode oscillation is required, the amount of horizontal misalignment between the center of a current confining aperture and the center of a surface relief structure is, for example, preferably 1 μm or less, and more preferably 0.5 μm or less.
This is because, if the centers are misaligned, an unnecessary loss is given to the mode in which the oscillation is required (fundamental transverse mode in this case), or, a necessary loss can not be given to the mode in which the oscillation is not required (high order transverse mode).
In H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999), as such a method of forming the surface relief structure and the current confining structure with the surface relief structure and the current confining structure being aligned with each other, a method called a self-alignment process is disclosed.
This method is characterized in that the positioning and patterning of a surface relief structure and a mesa structure are carried out at the same time.
By etching the mesa, a side wall of a selective oxidation layer is exposed, from which the selective oxidation layer is oxidized, to thereby form the current confining structure.
Therefore, the horizontal alignment between the surface relief structure and the current confining structure is performed automatically.
FIGS. 6A to 6E are schematic views for describing the self-alignment process disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999). FIGS. 6A to 6E describe a self-alignment process flow. As illustrated in FIG. 6A, a first resist 410 is applied to an upper mirror 114 of a wafer for a VCSEL, and the resist 410 is patterned at the same time in the shape of the surface relief structure and in the shape of the mesa structure. Here, a convex surface relief is illustrated.
Then, as illustrated in FIG. 6B, dry etching of the semiconductor is performed with the patterned resist 410 being used as a mask. The etching forms a surface relief structure 150.
Then, as illustrated in FIG. 6C, a second resist 420 is applied and patterned so as to protect the surface relief structure 150.
Then, as illustrated in FIG. 6D, wet etching is performed so as to form the mesa structure, and a high-Al-composition-ratio layer 115 is exposed at a side wall of the mesa.
Then, as illustrated in FIG. 6E, the resists 410 and 420 are removed and the high-Al-composition-ratio layer 115 is selectively oxidized to form a current confining structure 116.
From hereon, according to a standard process, an electrode is connected to the device to complete a VCSEL device.
By the way, in order to expose the selective oxidation layer by etching the mesa structure using the above-mentioned conventional self-alignment process such that the selective oxidation layer can be oxidized, the depth of the etching is required to be on the order of 3 to 4 μm.
In such deep etching, it is difficult to expose the side wall of the selective oxidation layer according to the position of the pattern of the mesa structure formed at the same time with the pattern of the surface relief structure. The reason is described in the following.
For example, when a deep mesa structure as described above is formed by wet etching, there are problems including difficulty in forming the mesa structure so as to be precisely vertical and liability to crystal orientation dependence of the semiconductor.
Further, when the deep mesa structure as described above is formed by dry etching, the resistance of the resist to the dry etching is low.
Therefore, there is a problem in that, because edge portions of the etching mask are damaged and pull back, the mesa structure can not be formed with high precision.
For those reasons, there is a possibility that the position at which the oxidation of the high-Al-composition-ratio layer starts (the position of the side wall exposed by the etching) is misaligned with the patterning position of the mesa structure.
In that case, the positions at which the oxidation starts are misaligned with the position of the patterned mask, and consequently, the size and the position of the current confining structure deviate from the size and the position that the current confining structure should have and become unstable.
As a result, as exemplarily illustrated in FIG. 7, there is a possibility that the surface relief structure and the current confining structure are not necessarily aligned with each other such that an effective single transverse mode VCSEL can not be obtained.